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Neuropsychiatr Dis Treat. 2010; 6: 159–195.
Published online 2010 May 6.
PMCID: PMC2874340

Cobalamin deficiency, hyperhomocysteinemia, and dementia

Abstract

Introduction

Although consensus guidelines recommend checking serum B12 in patients with dementia, clinicians are often faced with various questions: (1) Which patients should be tested? (2) What test should be ordered? (3) How are inferences made from such testing? (4) In addition to serum B12, should other tests be ordered? (5) Is B12 deficiency compatible with dementia of the Alzheimer’s type? (6) What is to be expected from treatment? (7) How is B12 deficiency treated?

Methods

On January 31st, 2009, a Medline search was performed revealing 1,627 citations related to cobalamin deficiency, hyperhomocysteinemia, and dementia. After limiting the search terms, all abstracts and/or articles and other references were categorized into six major groups (general, biochemistry, manifestations, associations and risks, evaluation, and treatment) and then reviewed in answering the above questions.

Results

The six major groups above are described in detail. Seventy-five key studies, series, and clinical trials were identified. Evidence-based suggestions for patient management were developed.

Discussion

Evidence is convincing that hyperhomocysteinemia, with or without hypovitaminosis B12, is a risk factor for dementia. In the absence of hyperhomocysteinemia, evidence is less convincing that hypovitaminosis B12 is a risk factor for dementia. B12 deficiency manifestations are variable and include abnormal psychiatric, neurological, gastrointestinal, and hematological findings. Radiological images of individuals with hyperhomocysteinemia frequently demonstrate leukoaraiosis. Assessing serum B12 and treatment of B12 deficiency is crucial for those cases in which pernicious anemia is suspected and may be useful for mild cognitive impairment and mild to moderate dementia. The serum B12 level is the standard initial test: 200 picograms per milliliter or less is low, and 201 to 350 picograms per milliliter is borderline low. Other tests may be indicated, including plasma homocysteine, serum methylmalonic acid, antiparietal cell and anti-intrinsic factor antibodies, and serum gastrin level. In B12 deficiency dementia with versus without pernicious anemia, there appear to be different manifestations, need for further workup, and responses to treatment. Dementia of the Alzheimer’s type is a compatible diagnosis when B12 deficiency is found, unless it is caused by pernicious anemia. Patients with pernicious anemia generally respond favorably to supplemental B12 treatment, especially if pernicious anemia is diagnosed early in the course of the disease. Some patients without pernicious anemia, but with B12 deficiency and either mild cognitive impairment or mild to moderate dementia, might show some degree of cognitive improvement with supplemental B12 treatment. Evidence that supplemental B12 treatment is beneficial for patients without pernicious anemia, but with B12 deficiency and moderately-severe to severe dementia is scarce. Oral cyanocobalamin is generally favored over intramuscular cyanocobalamin.

Keywords: Alzheimer, dementia, cognitive impairment, cognitive dysfunction, cobalamin, cyanocobalamin, B12, homocysteine, hyperhomocysteinemia, homocystinuria

Introduction

The notion of vitamin B12 deficiency (ie, B12 hypovitaminosis), its psychiatric and neurological manifestations, and its treatment is an age-old issue that has generated much controversy over many years. Although consensus guidelines14 recommend checking serum B12 levels in patients with dementia, clinicians are often faced with questions regarding how to interpret and what to do with the results.5 In order to help in clarifying these uncertainties, six questions were posed and then answered: 1) In which patients should vitamin B12 be routinely assessed? 2) What test or tests should be ordered, and how are inferences made from such testing? 3) Does the finding of low serum B12 or elevated homocysteine (Hcy) require evaluation for other medical conditions? 4) When vitamin B12 deficiency is found in dementia, is dementia of the Alzheimer’s type (DAT) a compatible diagnosis? 5) Based on the benefit-to-risk ratio in treatment of dementia, if vitamin B12 deficiency is determined, should supplemental B12 be initiated? 6) How is vitamin B12 deficiency treated?

By assimilating data from in vitro, in vivo animal, and in vivo human studies and epidemiological studies, this article clarifies these and other issues regarding hypovitaminosis B12, hyperhomocysteinemia (HHcy), and dementia.

Methods/results

The study design is a qualitative and quantitative review of the literature. The problem discussed is that in clinicians’ geriatric practices, patients with dementia and low vitamin B12 were not showing significant improvement with supplemental B12 therapy. Our hypothesis is that patients with dementia and low vitamin B12 improve with supplemental B12 therapy. The null hypothesis is that patients with dementia and low vitamin B12 do not improve with supplemental B12 therapy.

On January 31st, 2009 a Medline search was performed using the search terms: (Alzheimer OR Alzheimer’s OR dementia OR cognitive impairment OR cognitive dysfunction) AND (cobalamin OR cyanocobalamin OR B12 OR B-12 OR B 12 OR homocysteine OR hyperhomocysteinemia OR homocystinuria), which revealed 1,627 citations. “Title/Abstract” field limits decreased the search to 1,095 citations, which included 230 review articles. Using a Boolean operation, the review articles were removed, reducing the search to 865 citations. Subsequently, the search was limited to only citations with abstracts, so as to exclude publications such as ‘Letters to the Editor’ and case reports, which revealed 824 citations. Furthermore, in order to not miss any positive findings of individual case reports or case series, on September 6th, 2009 another Medline search was performed using the search terms: pernicious anemia AND dementia AND (case report OR case series) revealing 20 citations. Of the 844 articles, all abstracts were reviewed and, when useful, relevant articles were obtained and reviewed. Bibliographies from relevant articles were reviewed and, when applicable, review articles, ‘Letters to the Editor,’ and case reports were included in the overall review. Data from (1) other Medline searches, (2) Internet searches, (3) basic and clinical science textbooks, and (4) personal communications were added for clarification of technical issues. All abstracts, articles, and other references were categorized, allowing duplications, into six major categories: (1) general information, (2) biochemical evidence suggesting that hypovitaminosis B12 or HHcy are causal factors in dementia, (3) clinical and radiological manifestations, (4) associations between hypovitaminosis B12 or HHcy and cognitive impairment, (5) evaluation, and (6) treatment. Endnote version X.0.2 (Thomson Reuters, Philadelphia, PA) was used to maintain the reference library, which contained 839 citations. Evidenced-based medicine was used to develop suggestions for vitamin B12 workup and treatment in patients with suspected mild cognitive impairment (MCI) or dementia.

There are 511 articles and other references relevant to the six questions posed in the introduction and the six major categories listed above. Question 5 in the Introduction considers treatment benefits and risks. In terms of treatment benefits, the study objective was to determine whether or not vitamin B12 is beneficial for B12-deficient dementia. Letting the null hypothesis be, “Patients with dementia and low vitamin B12 do not improve with supplemental B12 therapy,” not rejecting the null hypothesis when it is not true would be a type 2 error (false negative). In order to avoid a type 2 error, thus concluding B12 treatment is not beneficial, when in truth it is beneficial, all published studies and reports contained in Medline (Box 1), including case series, in which supplemental B12 was an exposure and cognitive change was an outcome are included in the discussion and tables (N = 38), regardless of the quality of the study or number of subjects. Also included are all published cohort and longitudinal studies in Medline, where exposure pertains to metabolic or serum B12 deficiency and outcomes pertain to change in cognitive function or development or prevention of dementia (N = 37). Also included are the majority of the retrospective and cross-sectional studies in Medline that examine similar outcomes and exposures. Articles pertaining to genetics, biochemistry, pathophysiology, clinical manifestations, and radiological manifestations illuminating our understanding on relationships between HHcy with and without cobalamin deficiency and dementia are included in the review, as are articles relevant to evaluation, prognosis, and treatment.

Box 1. Studies and reports in which supplemental B12 is an exposure and cognitive change is an outcome

Abyad, 2002Fox, 1975McMahon, 2006
Aisen, 2008Healton, 1991Nilsson, 2001
Bolaman, 2003Hvas 2004Osimani, 2005
Bryan, 2002Ikeda, 1992Remington, 2009
Carmel, 1995Kalita, 2008Seal, 2002
Clarke, 2003Kwok, 1998Stott, 2005
Crystal, 1994Kwok, 2008Sun, 2007
Cunha, 1990La Rue, 1997Teunisse, 1996
Cunha, 1995Lehmann, 2003van Asselt, 2001
Eastley, 2000Levine 2006van Dyck, 2008
El Otmani, 2008Lin, 2008van Uffelen, 2008
Eussen, 2006Lindenbaum, 1988Wolters, 2005
Fourniere, 1997Martin, 1992

Discussion

General information

Vitamin B12 is composed of a central cobalt atom, attached to a dimethylbenzimidazole group, four nitrogen atoms, each pertaining to four pyrrole rings, and an R group (-CN, -OH, -CH3, or adenosyl group), denoting the specific type of cobalamin.68 The definition of vitamin B12 deficiency is a quantitative lack of vitamin B12 in the diet, body fluids, or cells or a qualitative lack of intracellular B12 utilization.

B12 deficiency occurs in roughly 10% of general923 and 17% of demented10,2426 elderly populations. B12 deficiency in demented individuals ranges from one10,27,28 to five25,29 times that of controls. Serum B12 levels and cerebral spinal fluid (CSF) folate levels decrease with advancing age,912,3034 whereas serum folate levels may either increase or decrease with age.32 Hcy is a nonessential thiol amino acid.35,36 HHcy is defined as an abnormally high level of total Hcy in the plasma. HHcy occurs in 6% to 81% of individuals, depending on the population studied.3744 Causes of HHcy and B12 deficiency are listed in Table 1.7,18,19,39,4562 Further details regarding differential diagnoses are available on the Internet.62

Table 1
Causes of hyperhomocysteinemia and B12 deficiency

Biochemical evidence suggesting that hypovitaminosis B12 or hyperhomocysteinemia are causal factors in dementia

The means by which HHcy is involved in dementia may possibly be explained by aging and reduced vitamin B12 or folate supply versus demand ratio,63,64 with functional, structural, genetic, and nutritional determinants. Possible functional determinants include Hcy agonism of N-methyl-D-aspartic acid (NMDA) receptors, which causes excessive intracellular calcium influx and neuronal death65,66 and HHcy creating a state of hypomethylation in the pathogenesis of Alzheimer’s disease (AD),21,58,67 causing deoxyribonucleic acid (DNA) damage and apoptosis.6871 HHcy inhibits adult mammal hippocampal neurogenesis. 72 Hcy may compete with gamma-amino butyric acid (GABA) at the GABA receptor and may affect its inhibitory function.73,74

Potential structural determinants include HHcy causing blood–brain barrier (BBB) dysfunction7578 and endothelial cell toxicity,47,64 whereby Hcy changes endothelial cell surface properties from anticoagulant to procoagulant.47,52,73,79 Genetic determinants include various polymorphisms and homozygous monogenic deficiencies, involving methylenetetrahydrofolate reductase (MTHFR), trans-cobalamin (TC) II, methionine synthetase (MS), and cystathione beta-synthetase (CBS). These may contribute to hypovitaminosis B12, HHcy, and dementia.7,47,52,64,73,8082 Meta-analyses8389 of the MTHFR polymorphism show that those with 677 TT alleles compared to 677 CC alleles have elevated Hcy and increased risk for myocardial infarction (MI), transient ischemic attack, and stroke. Hyperhomocysteinemic individuals with certain butyrylcholinesterase-K (BuChE-K) alleles cognitively decline more rapidly than those with wild-type BuChE alleles.90 Nutritional determinants include decreased vitamin B12 ingestion9,45,47 and food-cobalamin malabsorption.9,15,19,45,49,9193

In vitro, in vivo animal, and in vivo human studies suggest AD pathophysiology conceivably involves a hypomethylation state,67,9497 reactive oxygen species (ROS) generation,27,98101 immune activation,102110 and anomalous protein development.111 Abnormal Hcy metabolism is involved in each of these four processes. Hcy is metabolized by the methionine cycle and trans-sulfuration pathway (Figure 1),112 where it has one of three possible fates: methylation to methionine (Met), transsulfuration to cystathionine, or adenosylation to S-adenosylhomocysteine (SAH).113,114 With normal Hcy levels, the first two reactions are maintained, the first occurring in the brain and body and the second predominately occurring in the body, promoting homeostatic methylation reactions and maintaining healthy cells. With Hcy elevation the third reaction ensues, promoting a hypomethylation state leading to disease.115

Figure 1
Folate cycle, methionine cycle, and transsulfuration pathway. Copyright © 2005. Adapted with permission from Davis SR, Quinlivan EP, Shelnutt KP, et al. Homocysteine synthesis is elevated but total remethylation is unchanged by the methylenetetrahydrofolate ...

On the one hand, without oxidative chemical reactions, as the basis for cellular respiration, we would not have life, at least as we know it. On the other hand, without these reactions, we would not have ROS.116 There are thousands of publications related to ROS and aging, the greatest known risk factor for sporadic AD.114,116,117 Oxidative metabolism generates a very small fraction of ROS,117 which can be beneficial or detrimental to the central nervous system. Oxidative stress occurs when ROS generation exceeds ROS defense,118 leading to potential molecular and cellular damage.117 Aberrant mitochondrial enzymes may facilitate this process, thereby contributing to the pathophysiology of AD.116

Hcy is rapidly auto-oxidized to homocysteine thiolactone, homocystine, and mixed disulfides,7,119 producing ROS,18,21,47,52,68,120 including singlet oxygen, superoxide anions, hydroxyl radicals, and hydrogen peroxide.21,42,52,68,117,118 Hcy elevation is associated with microglia activation and proliferation71 and immune activation and deposition,108 which is associated with choroid plexus dysfunction,121123 possibly impeding vitamin B12123125 and folate122,125 influx to, and amyloid beta (Aβ) peptide123,126,127 clearance from, brain tissue in AD. In certain systems, Hcy elevation leads to amyloid precursor128 and tau128132 protein hyperphosphorylation, and Hcy oxidation produces products that crosslink with Aβ and tau proteins, causing their precipitation.119,128,133 Hcy elevation is associated with increased Aβ peptide, in both brain134 and plasma.51,135,136

A deleterious cycle may occur between ROS generation and immune activation, where the former may cause the latter137139 and vice versa.63,64,140144

Hcy may promote a means for such a cycle,64,106,120 because ROS generation is associated with Hcy elevation,120,142,145147 which is associated with immune activation,64,71,79,148150 and immune activation is associated with Hcy elevation,64,142,151,152 which is associated with ROS generation (Figure 2).47,64,71,153155 Additional evidence verifies ROS,145,151,156 hypomethylation,21,58,67,94,95,128,136,157 immunological components,64,79,106,109,110,142 and Aβ and tau proteins58,65,66,120,136,158 interact with one another, producing a neurodegeneration cascade in AD. ROS generation precedes both Aβ peptide deposition159162 and tau-associated neurodegeneration.163 ROS generation deregulates tau protein phosphorylation,164,165 and is associated with decreased S-adenosylmethionine (SAM),120 increased SAH, a decreased SAM/SAH ratio,115 and hyperconsumption and depletion of antioxidants98,151,156 and tetrahydrofolate (THF).64,120,151,156,166

Figure 2
Illustration of a biologically plausible deleterious cycle of reactive oxygen species (ROS), homocysteine (Hcy), and immune activation that possibly may be involved in the pathogenesis of Alzheimer’s disease.

With absolutely or relatively low folate or vitamin B12, MS-mediated Hcy clearance is impeded,52,120 resulting in a hypomethylation state. Also, Hcy elevation may lead to hyperconsumption and depletion of vitamin B12 in various cases of AD and vascular dementia (VaD).146 Hcy potentiates Aβ peptide-induced ROS generation and apoptosis.69,154,155,167 Aβ and tau proteins are concentrated sources for further ROS generation, Hcy elevation, and immune activation, thereby perpetuating the deleterious cycle.120,143,168170 Also, in vivo human studies show that free cobalt is elevated in individuals with AD compared to controls.8 In vitro studies show free cobalt generates oxidative stress, as measured by reduced glutathione, increases Aβ peptide secretion, and produces neuroblastoma cytotoxicity.8,120

Within the central nervous system, a balance may occur between endogenous neurotoxic agents on one hand and endogenous neurotrophic agents on the other hand.171 Examples of potential neurotoxic agents include tumor necrosis factor-alpha (TNF-α), nerve growth factor (NGF), and the soluble CD40-soluble CD40 ligand dyad (sCD40-sCD40). Examples of potential neurotrophic agents include interleukin-6 (IL-6), epidermal growth factor (EGF), and transforming growth factor-beta1 (TGF-β1). In animals, if the balance is tilted in favor of TNF-α, NGF, and sCD40-sCD40 (eg, by the administration of exogenous TNF-α),172 as opposed to IL-6, NGF, and TGF-β1, then the morphological changes of subacute combined degeneration (SCD) are observed: white matter interstitial edema, intra-myelinic edema, spongy vacuolation, and astrogliosis.172175 Vitamin B12-depleted animals exhibit increased levels of TNF-α, NGF, and sCD40-sCD40172,176 and decreased levels of IL-6 and EGF,176 thereby tilting the balance and developing the myelopathic changes of SCD. Not only does treatment with vitamin B12 reduce or reverse these changes,172,175 but treatment with anti-TNF-α antibodies, IL-6, EGF, and TGF-β1 does so as well.172,176 Interestingly, a similar observation has been observed in humans. Serum TNF-α is higher and serum EGF is lower in subjects with severe B12 deficiency compared to controls, where a direct correlation is found between plasma Hcy and serum TNF-α.173 The association is translative into the CSF, where CSF B12 and EGF are lower and CSF Hcy and TNF-α are higher in subjects with SCD compared to non-B12 deficient controls.177 B12-repletion lowers serum TNF-α and raises serum EGF, thereby normalizing the imbalance, which occurs concomitantly with clinical and hematological disease remission.173 Hence, in addition to well-known enzymatic roles for vitamin B12, it is thought to also have nonenzymatic roles, where it is associated with downgrading synthesis and release of TNF-α and upgrading synthesis and release of EGF.173,177,178

Clinical and radiological manifestations

Clinical manifestations of low vitamin B12 include abnormal psychiatric, neurological, and gastrointestinal findings. Psychiatric manifestations consist of psychoses, including paranoia, delusions, and hallucinations,23,58,179184 cognitive dysfunction, including memory impairment, delirium, and dementia,7,21,23,49,58,93,180,181,185188 and affective syndromes, including mania and depression,7,21,23,58,93,180,181,186,189191 which also occurs with elevated Hcy192194 and low SAM.67,192 Associations exist between HHcy and cognitive dysfunction in bipolar disorder195197 and perhaps schizophrenia.198 Neurological manifestations include myelopathy and peripheral, autonomic, and optic neuropathies.199 Paresthesia is caused by a sensory lesion anywhere between the peripheral nerve and brain and is often the initial symptom.23,58,92,180,181,188 SCD refers to myelopathy affecting posterior and lateral columns, characterized by a pernicious sequence of vacuolar demyelination, axonal degeneration, and neuronal death.18,58,200202 Posterior column myelopathy affects afferent pathways, causing the most common neurological signs: ataxia, diminished proprioception and vibratory senses, and presence of Romberg’s sign.18,49,58,180,185,188,200,201,203 Lateral column myelopathy affects efferent pathways, causing the second most common neurological signs: extremity muscular weakness, spasticity, hyperactive reflexes, and Babinski’s sign.18,49,58,180,181,200,201 Peripheral and autonomic neuropathies cause hypoactive reflexes, sensory loss, orthostatic hypotension, fecal and urinary incontinence, and impotence. 18,23,49,58,92,180,181,185,188,200,203 Although optic neuropathy is uncommon,23,58,180,203 visual impairment may occasionally be the earliest or sole manifestation of the disease.203 Gastrointestinal manifestations include epithelial atrophy of the tongue, referred to as atrophic glossitis, which causes the tongue to be sore and beefy red,18,23 and epithelial atrophy of the stomach.15

Computerized axial tomographic (CAT) and magnetic resonance imaging (MRI) scans of nondemented elderly brains may show age-related cerebral atrophy and various grades of periventricular white matter disease consistent with chronic microvascular ischemia.204208 Although linear and volume measurement methods, evaluating ventricle-to-brain ratios and medial temporal lobe atrophy, reveal significant differences in group means, between those with AD and controls,205,207,209217 such strategies are not recommended for the purpose of diagnosing AD.3,218 Assuming normal distributions of both nondemented and demented groups, a certain degree of overlap may exist,205,207,216 unless specified by scan angle-adjusted temporal lobe neuroimaging.207,214 Even so, such imaging may not distinguish non-Alzheimer’s dementia from controls.217 Although CAT scans of nondemented elderly commonly show age-related cerebral atrophy, those of demented elderly often show cerebral atrophy more than expected for age,210 and are read as variably judged atrophy and differently interpreted white matter changes.218,219 Whether or not such findings relate to dementia with low vitamin B12 and/or elevated Hcy is often unclear.

Accordingly, a literature review finds radiological manifestations of low vitamin B12 and/or elevated Hcy to include leukoaraiosis, brain atrophy, and silent brain infarcts.199,220237 Leukoaraiosis is a radiological term that refers to brain white matter hypodensity on CAT scans or hyperintensity on T2-weighted magnetic resonance imaging (MRI).238 It is associated with aging,221,230,239244 chronic microvascular hypo-perfusion, 245 BBB dysfunction,246 hypertension,221,230,240,241,242,244 stroke,230,239,240242,244,247 and death.239 Leukoaraiosis is described pathologically as periventricular leukoencephalopathy or subcortical arteriosclerotic encephalopathy; it occurs in brains of individuals with AD243 and dementia of the Binswanger type (DBT),248 which is a relatively rarer type of dementia and is associated with other findings. HHcy increases the risk for leukoaraiosis.220,221,223226,228,230,232,236 Independently from plasma Hcy levels, one cross-sectional study249 found an inverse association between the concentration of normal range serum B12 levels and the degree of leukoaraiosis; nonetheless, in the absence of HHcy, two studies227,231 found low serum B12 does not increase the risk for leukoaraiosis. Parenthetically, one prospective study250 found an inverse association between serum folate levels and the degree of leukoaraiosis. Although leukoaraiosis is more prevalent in demented than nondemented individuals225,247 and it increases the risk for developing dementia,247,248,251 results are mixed in terms of whether or not the link between HHcy and cognitive dysfunction is specifically mediated by leukoaraiosis.228,232,240,243,252255 In both cross-sectional and prospective evaluations, hypovitaminosis B12 and HHcy are associated with brain atrophy.199,222,229,232,234,237 HHcy is associated with silent brain infarcts.232

VaD,61 DBT,256 and AD,61,222,243,257259 represent various spectra of vascular pathology. To illustrate, cerebral amyloid angiopathy occurs in many cases of DBT248 and in most cases of AD,46,260,261 where increased vessel atrophy,262 decreased microvascular density,262 reduced temporal lobe blood flow,263,264 and spontaneous cerebral emboli265,266 are other significant findings. Hcy permeates through the BBB,267,268 causing BBB dysfunction,74,7678 which is observed in AD,247,269 DBT,220,223,256,270 and VaD,75 allowing easy influx for a wide variety of proteins to cerebral interstitial fluid271,272 and vice versa.273 With BBB dysfunction, brain parenchyma likely becomes less protected from toxic effects of systemic HHcy,229 which increases risk for acute macrovascular disease, or strokes,7,21,70,84,85,87,151,223,274279 causing loss of volume, and chronic microvascular disease, or ischemia,135,220,280 causing loss of cortical-to-subcortical connections,248 occasionally with findings as those in DBT.220,223,281,282

Associations between hypovitaminosis B12 or HHcy and cognitive impairment

HHcy, with hypovitaminosis B12, are common findings in the evaluation of MCI,59,283288 AD,43,60,61,82,111,222,276,284,289291 VaD,43,61,145,276,284,292 and other dementia subtypes.90,282,293,294 This raises the question of whether or not these findings are the cause of, result of, or unrelated to the disease process. Although multiple retrospective and cross-sectional studies find associations between hypovitaminosis B12 and HHcy and cognitive impairment with or without dementia,28,39,43,5961,82,90,111,145,186,196,222,276,283306 cohort studies are required to provide evidence that hypovitaminosis B12 or HHcy are risk factors for cognitive dysfunction. Eighteen106,136,252,258,301,307320 of 22 cohort studies demonstrate HHcy either increases the risk for cognitive impairment or development of dementia. Depending upon whether plasma Hcy levels increase or decrease over time, the degree of change either increases the risk for developing dementia313 or decreases the risk for poorer memory performance,316 respectively. The greater the baseline plasma Hcy level, the faster the rate of cognitive decline.320 However, one study found that the elevated plasma Hcy risk for developing dementia diminished when controlling for low serum folic acid.301 Four321324 of 22 cohort studies conclude Hcy is not a factor in the development of MCI or dementia, although outcome assessment may have been a methodological weakness in one of these.324 One cohort study325 concludes HHcy is a consequence of the development of cognitive impairment. Most,222,276,316,326328 but not all,329 studies find HHcy is associated with intensity, rather than duration, of illness in AD. Thus, evidence that HHcy increases the risk for cognitive impairment and dementia usually is consistent and reproducible, 233 with HHcy predictably increasing the risk that subjects with MCI will progress to dementia.233 With respect to the involvement of low vitamin B12, seven cohort studies308,312,313,330333 show low vitamin B12 either increases the risk for cognitive impairment or development of dementia, while eight13,325,334339 show no increased risk. Paradoxical findings in these studies include modest increases in serum B12 levels over time may increase the risk for dementia313 and individuals with normal serum B12 levels having a higher incidence of AD compared to those with low serum B12 levels. 13 Thus, evidence that low vitamin B12 increases the risk for cognitive impairment or dementia is inconclusive. Then again, the increased risk may occur in the opposite direction. AD may increase the risk for B12 deficiency.340

Evaluation

In which patients should vitamin B12 be routinely assessed?

Guidelines for the workup of dementia are listed in Figure 3.4,341 Although practices may range from checking serum B12 in all elderly to only particular patients with dementia, an evidence-based approach warrants checking serum B12 in all patients with MCI and mild to moderate dementia of two years or less duration. It is especially useful to assess serum B12 levels in all patients with MCI, because many will ultimately advance to dementia, where there may be a window of opportunity when vitamin B12 treatment of B12 deficiency-related cognitive dysfunction is potentially beneficial. Additionally, serum B12 should be checked in all patients with (1) history of gastric bypass surgery, partial or total gastrectomy, terminal ileum disease or resection, or pancreatic insufficiency,14,20,33,200 (2) chronic use of levodopa, histamine type-2 (H2) receptor blockers, or protein pump inhibitors (PPIs),22,33,51,54,342345 or (3) findings suggestive of (a) behavioral and psychological symptoms of dementia (BPSD), (b) SCD, including paresthesias, ataxia, or loss of position or vibratory senses, or (c) pernicious anemia (PA), including low hemoglobin (Hgb), elevated mean corpuscular volume (MCV), or corpuscular changes on peripheral smear.62 If such findings are absent, and patients have moderately severe to severe dementia of longer than two years duration, then universal recommendations14 of assessing for and, when found, treating B12 deficiency may not be supported by reliable evidence.13,16,24,26,92,187,326,346364 H2 receptor blockers and PPIs impair cobalamin absorption; 18,22,33,365,366 their use is associated with supplemental B12 initiation.367 Individuals who have had gastric surgery have a high prevalence of B12 deficiency,20 and those who have had gastrectomies, who are B12-deficient, have a high prevalence of cognitive dysfunction and electroencephalographic (EEG) abnormalities.332 Therefore, patients prescribed H2 receptor blockers or PPIs, and those who have had gastrectomies or gastric bypass surgery, require monitoring for hypovitaminosis B12.368

Figure 3
Guidelines for the workup of dementia.

What test or tests should be ordered, and how are inferences made from such testing?

Common tests include the deoxyuridine suppression test (dUST), serum B12, serum TC II, plasma Hcy, serum or urinary methylmalonic acid (MMA), and the post-methionine load test. For convenience, conversion of vitamin B12 units (nanograms per liter to picograms per milliliter (pg/mL) and pg/mL to picomoles per liter) are available at http://www.cdc.gov/ncbddd/b12/index.html.

The dUST is probably the most sensitive and specific test for assessing functional folate or B12 deficiency.92,369 Although the test is not fully understood and probably more complex than a simple explanation,369 if incubated bone marrow cells or peripheral blood lymphocytes have sufficient folate and cobalamin, nonradioactive deoxyuridine is believed to suppress the thymidylate synthetase conversion of radioactive thymidine into thymidylate, which is a normal response, but if cells have insufficient folate or cobalamin, nonradioactive deoxyuridine does not suppress the thymidylate synthetase conversion of radioactive thymidine into thymidylate, which is an abnormal response.

The most common test is the serum B12 level,15 but often it is difficult to determine which serum B12 levels represent deficiency states and which do not. When using a specific serum B12 cutoff point, for example 200 pg/mL or less,22,181,200,368 in determining which individuals do and do not have B12 deficiency, problems encountered include false positives and false negatives.7,15,19,30,33,57,188,370376 One of the reasons for this is because most serum B12 assays measure total B12, including free B12 and that bound to B12 binding proteins: TC I, also called haptocorrin, TC II, simply referred to as transcobalamin, and TC III, which is produced by neutrophils.7,371,374 TC I and III are R-proteins (R for rapid movement on electrophoresis).7,23,200,377 TC I is a storage protein and does not participate in cellular uptake.200,377 TC II participates in all cellular uptake.7,58,200,371,372,377 TC III participates in hepatocyte uptake only.200 TC I and II comprise about 80% and 20% of total B12, respectively. Falsely low serum B12 occurs in congenital TC I deficiency, where TC I is low or absent, but TC II, intracellular B12, and hematopoiesis are normal.7,56,200,377 Other causes of falsely low serum B12 include folate deficiency, multiple myeloma, oral contraceptive use, and pregnancy.7,56 The mechanism by which low serum folate causes falsely low serum B12 is poorly understood.7,368 Alternatively, low serum cobalamin causes methyltetrahydrofolate (CH3-THF) trapping, elevating serum CH3-THF, which causes falsely high serum folic acid (Figure 1).58,200,372,378 Falsely normal serum B12 occurs in congenital TC II deficiency, where TC I is normal, but TC II is low or absent, and intracellular B12 is insufficient, causing severe megaloblastic, macrocytic anemia.7,56,200,377,379 Other causes of falsely normal serum B12 include liver disease, myelopro-liferative disorders, and intestinal bacterial overgrowth.7,56 Since serum TC II levels decrease with advancing age, a given serum B12 level in an elder may represent a deficiency, compared with the same level in a younger adult.30,380 Directly measuring serum TC II may be helpful in these cases and others.17,38,308,335,351,372,374,380383

Studies show 10%56,203 to 50%19,368 of truly vitamin B12 deficient individuals have serum B12 between 200 and 350 pg/mL, presenting clinicians with a diagnostic challenge. Based upon known biochemistry, specific tests can be ordered to help with this challenge. Cytoplasmic methylcobalamin is needed for MS-catalyzed Hcy methylation to Met, and mitochondrial adenosylcobalamin is needed for L-methylmalonyl-CoA mutase-catalyzed L-methylmalonyl-CoA conversion to succinyl-CoA.7,18,19,20,52,57,58,203 With low cellular B12, these reactions are impeded, elevating Hcy in the former and L-methylmalonyl-CoA, D-methylmalonyl-CoA, and MMA in the latter.18,20,285 Thus, HHcy and hypermethylmalonic acidemia (HMMA) are surrogate markers for low vitamin B12 cellular levels (ie, metabolic B12 deficiency).7,11,15,1922,30,33,44,59,60,188,203,285,294,351,353,368,370,372,373,376,380,384,385 HHcy occurs with deficiency of pyridoxine, folate, or vitamin B12;19,306,355,370,386 usually, only deficiency of vitamin B12 causes HMMA.19,20,370 Hcy may be the more sensitive and MMA the more specific surrogate marker.17,20,38,40,151,156,285,387389 Either surrogate marker is more sensitive than serum B12 in evaluating PA,7,19,373,390 and many individuals with HHcy have normal serum folate and B12 levels.38,39,42 When used alone, B12,371,375 TC II,371 Hcy,38 or MMA389 may be insufficient as screening tests, but in combination, normal plasma Hcy and serum MMA rule out B12 deficiency in virtually all cases.19,390 Although the post-methionine load test is twice as sensitive as basal Hcy in detecting HHcy in individuals with AD,391 it is not recommended as an initial test.17

Most laboratory assays measure total Hcy, including reduced and oxidized forms: Hcy representing the former and homocystine and mixed disulfides (protein-bound Hcy and cysteine-Hcy) the latter.52 Although the upper reference limit for plasma Hcy varies according to different conditions,17,33 plasma Hcy greater than 14 μmol/L in patients with vitamin B12 levels between 201 and 350 pg/mL suggests cellular B12 deficiency.23,41,200,222,252,258,357,387,392 Serum B12 greater than 350 pg/mL rules out B12 deficiency in almost all individuals.56,200,340 Since tissues store vitamin B12 for up to five years18,368,393 and plasma Hcy increases only minimally after protein-rich meals,17 serum B12 and plasma Hcy may be obtained fasting or nonfasting. Immediate centrifugation, or keeping samples refrigerated or ice cooled until centrifugation, prevents spuriously elevated Hcy results.17,394,395 Suspected B12 deficiency can be confirmed when individuals having characteristic findings, including anemia, macrocytosis, corpuscular changes on peripheral smear, or signs and symptoms of SCD or peripheral neuropathy, improve with vitamin B12 treatment or when elevated Hcy or MMA are lowered with vitamin B12 treatment.19,20,92,135,188,347,362,370,392,396,397 Although not intended to replace clinical judgment, suggestions for vitamin B12 workup and treatment in patients with suspected MCI or dementia are presented (Figure 4).

Figure 4
Suggestions for vitamin B12 workup and treatment in patients with suspected neuropathology.

Does the finding of low serum B12 or elevated Hcy require evaluation for other medical conditions?

Some authors recommend establishing the etiology of B12 deficiency as part of the diagnostic approach.5 It is beyond the scope of this paper to cover all diagnostic considerations in Table 1, but levodopa treatment, folate deficiency, and PA are noteworthy.

In Parkinson’s disease (PD) it is the treatment rather than the disease that causes HHcy.37,54,294 In such cases, HHcy may293,294 or may not398 be associated with cognitive impairment. Recall from Figure 1, Met is converted to SAM, which is converted to SAH, which is converted to Hcy. SAM demethylation to SAH serves as a methyl group donor for the biosynthesis of nucleic acids, proteins, phospholipids, and catecholamines and the metabolism of drugs and toxins. Catechol-O-methyl transferase (COMT) catalyzes levodopa methylation to 3-O-methyldopa.344 Increased methyl group demand, required for the methylation of levodopa to 3-O-methyldopa, favors the conversion of Met to SAM, demethylation to SAH, conversion to Hcy, and development of HHcy.51,54,342,344 In a cross-sectional study,342 individuals on levodopa alone, compared to those on combined levodopa and COMT inhibitor therapy, had higher plasma Hcy levels. However, only one344 of three54,343,344 prospective studies showed addition of a COMT inhibitor to those on levodopa prevents dopamine-associated plasma Hcy elevation or serum B12 reduction.

Since the relationship between folate and vitamin B12 is biochemically and, in deficiency states, pathologically united,285 there are caveats to keep in mind when working up and treating folate and B12 deficiencies. When folate and B12 deficiencies occur together, monotherapy with either folate or vitamin B12 can worsen the manifestations of the other vitamin deficiency.7,23,180,285,303,355,371,399,400 Thus, it is beneficial to assess serum levels of both vitamins and treat whichever deficiency occurs. Although PA and SCD share the common etiology, type A (autoimmune) chronic atrophic gastritis, with vitamin B12 malabsorption, they have distinct pathophysiologies.18,201 In the methionine cycle, folate- and vitamin B12-dependent MS catalyzes Hcy methylation to Met, as CH3-THF, serving as the methyl group donor, is demethylated to THF.47,52,58,151,200 Impairment of this reaction causes defective DNA synthesis, leading to megaloblastic, macrocytic anemia (ie, PA).21,58 Supplemental folic acid can override the impairment, restoring DNA synthesis and normalizing erythropoiesis.58,401 Furthermore, in the methionine cycle, Met is adenosylated to SAM, which is demethylated to SAH, which is deadenosylated to Hcy.58 Impairment of SAM demethylation to SAH impedes methylation reactions, leading to vacuolar demyelination, axonal degeneration, and neuronal death (ie, SCD).21,58 Supplemental folic acid cannot override the methylation impairment, resulting in progressive neuropathology and neuronal death.58 Treating a combined folate and B12 deficiency with folic acid alone may correct hematological abnormalities, but not neurological abnormalities, and can aggressively worsen B12-deficient neurological sequelae.7,18,19,23,200,203,355 Thus, B12 deficiency should be ruled out before correcting folate deficiency.

When folate and vitamin B12 are in balance, increased serum folate levels are associated with decreased plasma Hcy and serum MMA levels, but when vitamin B12 is underrepresented, increased serum folate levels are associated with increased plasma Hcy and serum MMA levels.285 Also, higher folate states require relatively higher vitamin B12 levels than normal folate states to protect against metabolic B12 deficiency.285 Therefore, the borderline (201–350 pg/mL) serum B12 range may be higher in the presence of high folate states.

In what was described as the most severe neuropathic epidemic of modern times,402 between 1991 and 1993 more than 50,000 Cubans developed peripheral neuropathy, associated with reduced nutrient intake of group B vitamins,403409 in the setting of strict embargos and economic deterioration.404,405,407,408 Although widespread distribution of group B vitamins404,406,407,409 and government-mandated folic acid fortification404,410 curbed the epidemic,404,406 some speculate that endemic subclinical group B vitamin deficiencies, coupled with Helicobacter pylori (H. pylori) infections, are responsible for the higher prevalence of dementia in Cuba compared with other Caribbean countries.404 Before and after the Cuban epidemic, in 1976 and 1995, researchers411,412 discovered that folate deficiency and relatively high, compared with relatively low, Hcy levels in pregnant women are associated with neural tube defects (NTDs) in infants born to such women, findings that have since been replicated.7,18,52,355,393,413,414 Since supplemental folic acid during pregnancy decreases the risk for such abnormalities, 415420 in 1998 the United States Food and Drug Administration (FDA) and Health Canada required wheat and other grain products to be fortified with folic acid.35,58,421423 To date, more than 50 countries have mandated folic acid fortification.410,420 It is largely accepted that high-dose folic acid may mask B12 deficiency. It is less clear whether or not low dose folic acid masks B12 deficiency.285,401 Although the United States FDA requires 140 μg of folic acid per 100 g of grain or flour,424,425 an amount chosen because it was considered high enough to prevent NTDs, but low enough to not mask B12 deficiency, some have advocated increasing this amount.426 Due to insufficient sensitivity, neither Hgb nor MCV are useful in ruling out B12-deficient dementia or SCD.7,13,33,58,124,289,300,371,373,378,385,427 Although the complete blood count (CBC) is part of routine diagnostic testing for dementia,368 additional vigilance is needed,20,58,381,428 especially in the era of folic acid fortification. In B12 deficiency, supplemental folic acid may protect against anemia, but not neurodegeneration;19,52,371 thus, the CBC alone may become even less sensitive in evaluating B12-deficient psychiatric and neurological abnormalities.

Further evaluation to rule out type A chronic atrophic gastritis, or PA,181,429 might be warranted because of increased risk for gastric cancer, where radiographic or endoscopic screening is useful, especially in cases of MCI and early dementia.15,18,33,200,201 Thus, it is useful to obtain additional testing when B12-deficient dementia occurs with anemia, macrocytosis, corpuscular changes on peripheral smear, or signs and symptoms of SCD or peripheral neuropathy.7 When B12-deficient dementia occurs in the absence of such findings, the decision to rule out type A chronic atrophic gastritis may be made on a case by case basis.

Although PA, or type A chronic atrophic gastritis,181,429 is often cited as the most common cause of low serum B12,7,18,19,33,49,56,58,180,201,390 emerging evidence suggests dissociation between PA and B12-deficient dementia.10,13,15,1719,21,30,32,33,45,4749,56,58,9193,146,180,190,200,201,203,290,350,368,371,373,376,393,427434 Thus, B12-deficient dementia may be subdivided into B12-deficient dementia with PA and B12-deficient dementia without PA. Studies support the postulate that these two disease states are different.10,1315,17,19,21,25,30,32,33,45,4749,58,9193,146,190,201,203,289,290,350,368,427,429,431435 They appear to have different etiologies, pathophysiological findings, prevalences, and responses to treatment.

B12-deficient dementia with PA and B12-deficient dementia without PA have separate etiologies, whereby the former is caused by type A chronic atrophic gastritis181,429 and the latter is generally caused by type B (nonautoimmune) chronic atrophic gastritis, age-associated decrease in hydrochloric acid production, or decreased vitamin B12 ingestion.9,10,14,17,21,22,30,45,47,49,58,91,92,203,368 Type B chronic atrophic gastritis and age-associated decrease in hydrochloric acid production cause food-cobalamin malabsorption.30 Less established causes of relative B12 deficiency include impairment of vitamin B12 transfer, from capillaries to CSF436 and from CSF to neurons,382 and vitamin B12 hyperconsumption,54,63,340,342 inactivation,146 or destruction.290 Type A chronic atrophic gastritis is due to antiparietal cell and anti-intrinsic factor antibodies,15,33,58,181,201,203,429 which occur in 90% and 55% of cases, respectively,7,203 causing parietal cell insufficiency and intrinsic factor dysfunction, sparing the antrum.33,58,201 Type B chronic atrophic gastritis is usually caused by H. pylori infection, involving the antrum.15,58,201,203 This is especially worth mentioning, because gastritis involving both body and antrum437 and H. pylori infection, confirmed by serological,437439 histological,437,438,440 and rapid urease438 tests, is significantly more prevalent in individuals with MCI437 and AD438440 compared to controls. In those with MCI, there is a correlation between serum anti- H. pylori immunoglobulin G concentration and degree of cognitive impairment,437 which may be mediated by elevated Hcy levels.437 One study438 showed H. pylori eradication improved both cognitive and functional measures in patients with AD. Since gastrin is secreted by antral G-cells, serum gastrin is elevated with type A,378 but low with type B,21 chronic atrophic gastritis.

B12-deficient dementia with and without PA have separate pathophysiological findings, where myeloneuropathy is characteristic and macrocytosis or anemia are present in the former,181,190,429 but myeloneuropathy may be rare19 and macrocytosis and anemia are often absent13,19,25,30,92,146,289,290,350,427,431433 in the latter. Neuropathy associated with PA classically involves disease progression, beginning in the cervicothoracic region of the spine,202,441 with upper extremity findings classically occurring before lower extremity findings, followed by peripheral nerve involvement, and lastly, if at all, brain involvement.23,200,203 Dementia is believed to be a late and rare finding of neuropathy associated with PA.19,25,48,203,290 Alternatively, dementia with B12 deficiency often occurs independently from PA.13,25,146,290,427,432,433

B12-deficient dementia without PA is relatively common,25,146,290,427,432,433 but B12-deficient dementia with PA is considered rare,14,19,25,30,32,48,93,203,290,427,432 where it causes only a small fraction of B12 deficiency in demented individuals.14,25,30,32,93,290,427,432 PA incidence peaks late in midlife,181,429 while dementia incidence increases with age.233 Both chronic atrophic gastritis type A and B are characterized by low pepsinogen I/pepsinogen II ratios and achlorhydria;7,15,21,22,33,378 however, low pepsinogen I/pepsinogen II ratios occur in roughly 30% of elderly populations378 and PA occurs in only about 2%,23,428 suggesting the majority of cases with low ratios are caused by factors other than PA.

Meta-analyses or systematic reviews confirm that HHcy is a risk factor for AD,111,233 but they also confirm that vitamin B12 treatment of B12-deficient dementia is ineffective for improving cognitive function.16,348,355 Thus, B12-deficient dementia without PA and B12-deficient dementia with PA have distinct responses to supplemental B12 treatment, where it appears that moderately severe to severe dementia does not improve in the former,13,15,16,24,26,33,92,187,326,349,350,355,356,358,359,362 but may improve in the latter.185,190,199,429,434,435 The apparent dissociation of hematological findings and neurocognitive impairment provides additional support to the postulate that B12-deficient dementia with or without PA represent two separate disease states. Progressive neurocognitive decline often occurs in the presence of normal Hgb and MCV, where one or both values are normal in many individuals with B12-deficient dementia13,33,124,188,200,433 or neurological abnormalities.18,20,124,180,188,373 When anemia is absent, Hgb is indirectly proportional to cognitive function,18,19,200 and when present, it is an inconsistent risk factor for cognitive dysfunction.92,430 Neurological impairment occurring concomitantly with anemia is more likely to improve with supplemental B12 therapy in those cases where the anemia is more severe and less likely to improve where the anemia is less severe.180,393 With supplemental B12 therapy, increased reticulocytosis occurs within one week,376,393 but neurological improvement may require six months or longer.180,393 Vitamin B12 supplementation reverses hematological abnormalities in almost all patients,15,33,185,362 may improve neurological abnormalities in roughly one-half,15,33,185 and arrests or reverses dementia in only very few.13,15,16,24,26,33,92,187,326,347,349,350,355,361,362

In B12 deficiency dementia with versus without PA, there appear to be different manifestations, need for further workup, and responses to treatment. Therefore, when findings include low serum B12 or elevated Hcy, it is useful to rule out PA. In the workup for type A chronic atrophic gastritis, assessment of antiparietal cell and anti-intrinsic factor antibodies is an initial option.32,181,368,429 Since the former are sensitive, occurring in up to 10% of elderly populations,56 and the latter are specific, the absence of antiparietal cell antibodies suggests low likelihood, 7 while the presence of anti-intrinsic factor antibodies suggests high likelihood,7 for type A chronic atrophic gastritis. If antiparietal cell antibodies are present and anti-intrinsic factor antibodies are absent, assessment of serum gastrin levels is a further option.7,33 Type A and B chronic atrophic gastritis also can be distinguished by endoscopy with mucosal surface biopsy.7,181,429 Although Schilling’s test has been largely supplanted by the aforementioned tests,203,368 it may be useful if such tests are unremarkable or equivocal.434 However, decreased vitamin B12 ingestion9,47 and food-cobalamin malabsorption9,45,49 are common causes of low serum B12 in elderly populations, and Schilling’s test is most often negative (ie, step one results are normal) in these conditions.25,33,93,203

When vitamin B12 deficiency is found in dementia, is dementia of the Alzheimer’s type a compatible diagnosis?

AD is diagnosed by tissue pathology. According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR), DAT is essentially a diagnosis of exclusion, after other causes of dementia, including B12 deficiency, have been ruled out, thus dichotomizing dementia due to B12 deficiency and DAT. Although two studies359,442 support this dichotomy, most studies13,26,82,222,333,336,382 suggest DAT occurs with or without B12 deficiency. When MCI and various subtypes of dementia are compared with one another, similar concentrations of serum B12 and Hcy are observed.276,284,443 When B12-deficient dementia is compared to other subtypes of dementia, similar rates of decline are observed across almost all measured outcomes.336,350,444 Nonetheless, there is likely a DAT subgroup, having low serum B12427,445 and perhaps elevated platelet monoamine oxidase (MAO) activity.29 Since B12 deficiency dementia with PA may arrest or reverse with treatment190,429,434 and B12 deficiency dementia without PA is usually progressively neurodegenerative despite treatment, the presence of PA would suggest dementia due to B12 deficiency, while its absence would suggest DAT.446

Treatment

Based on the benefit-to-risk ratio in treatment of dementia, if vitamin B12 deficiency is determined, should supplemental B12 be initiated?

Vitamin B12 treatment is considered one of the safest medical treatments available.447 Although severe adverse events are very rare, those reported include anaphylactic shock and death with administration of parenteral vitamin B12, hypokalemia with sudden death in conditions of severe anemia, and severe and sudden optic atrophy in those with Leber’s hereditary optic neuropathy.448 Other adverse events include polycythemia, thrombosis, pruritus, rash, skin eruptions449 congestive heart failure, diarrhea, edema, and swelling. One randomized controlled trial (RCT)347 found a greater prevalence of depression in the treatment group, who received high-dose pyridoxine, folate, and vitamin B12, than in the placebo group.

Although no overdosage has been reported, in treating hypovitaminosis B12 with supplemental B12, there is a fine line between help and harm. The serum cobalamin-plasma Hcy concentration–response curve appears curvilinear.17 As serum cobalamin increases from the lowest detectable level to 950 pg/mL, plasma Hcy decreases, but as serum cobalamin further increases from 950 pg/mL to 1,350 pg/mL, plasma Hcy begins to rise. One cohort study,386 found a dose-dependent association between increasing serum B12 levels and incidence of coronary artery disease (CAD) and mortality, where each 100 pg/mL increase in serum cobalamin was associated with a 10% increased incidence for such events. One RCT450 found that low-dose pyridoxine, folic acid, and vitamin B12 treatment decreased the risk for stroke, CAD events, and death, while moderately high dose pyridoxine, folic acid, and vitamin B12 treatment did not. Although the earliest two451,452 of eight other RCTs showed combined pyridoxine, folic acid, and vitamin B12 treatment decreased morbidity in individuals with or at risk for CAD, the later six453458 failed to validate these findings, and four454,456458 of the six showed treatment potentially increased the risk for harm. One of these RCTs455 was prematurely terminated due to potential risk for harm demonstrated by group B vitamin treatment.454 Therefore, some authors386,454 suggest supplemental B12 should not be administered unless B12 deficiency is present, and when present, only enough supplemental B12 should be administered to correct the deficiency. Parenthetically, meta-analyses of folic acid treatment, aimed at lowering HHcy, show mixed results in terms of whether or not treatment decreases the risk for stroke.459,460 Since most prospective studies show supplemental B12 does not improve cognitive function or prevent dementia in cognitively intact individuals,306,354,360 including those with hypovitaminosis B12351,353 or HHcy,357 it is not recommended401 for these purposes.

HHcy increases the risks for birth defects, cognitive dysfunction, dementia, cerebrovascular disease, stroke, CAD, MI, peripheral vascular disease, venous thrombosis, osteoporosis, hip fractures, and death.21,39,279,386,461468 Benefits of treating B12 deficiency include lowering Hcy levels,19,21,47,135,198,326,347,351,357,360362,397,469 lowering Aβ peptide levels,470 decreasing tau hyperphosphorylation,132 anti-inflammation, 471 neuroprotection,471 reduction of MAO activity,29 and causing the BBB to be less leaky.469

It may not be feasible to perform a meta-analysis on studies that have ascertained whether or not vitamin B12 treatment is associated with improved cognitive function or prevention of dementia due to the heterogeneity of these studies (Table 2). Published studies include multiple designs (eg, case-control, other retrospective studies, RCTs, and other prospective studies), subjects (eg, those who are healthy, ill, communitydwelling, residents of tertiary care facilities, with and without cognitive dysfunction, and with and without vitamin B12 deficiency), interventions, including vitamin B12 alone or in combination with other drugs or dietary supplements, type of vitamin B12 administered, and route of administration.

Table 2
Examples of multiple binary variables in studies examining efficacy of B12 treatment on cognition

Traditional recommendations of assessing for and, when found, treating B12-deficient dementia are supported by case-control studies,350,359 retrospective series,49,180,188,435,472 and case reports,190,429,434 where B12 deficiency was specifically caused by PA. Evidence supporting these recommendations appears to diminish with many RCTs,16,185,198,298,306,326,347,348,351353,355,357,360,361,364,473,474 clinical trials (CTs),13,24,185,187,199,346,356,362,397,469 and other prospective studies26,92,349,358,475 (Tables 3a and b). Twelve16,306,326,347,351353,355,357,360,361,364 of 16 RCTs found supplemental B12 therapy was of no benefit for improving cognitive function. Four185,198,473,474 of 16 RCTs found supplemental B12 therapy was beneficial for improving cognitive function; however, subjects in these studies included nondemented, healthy adult women,473 individuals with schizophrenia and HHcy,198 and individuals with dementia, irrespective of serum B12 levels, treated with multiple vitamins and other dietary supplements.474 The design of one of these studies185 did not include a separate control group. In all other RCTs of individuals with hypovitaminosis B12,16,351,353,355 HHcy,357 HMMA,352 dementia,16,326,347,355,361 or at risk for dementia,326,348,352,360,364 vitamin B12 treatment group results were no better than placebo group results. Nonetheless, the results from the RCTs may not generalize to all patients. Five of these306,351,353,357,360 did not examine subjects with MCI or dementia, three16,326,355 may have included subjects in various dementia stages, and six appear to have enrolled subjects, with either normal347,361 or irrespective306,326,360,364 of baseline folate and vitamin B12 statuses.

Table 3a
Studies showing vitamin B12 treatment is not associated with improved cognitive function or prevention of dementia

Although few included control groups,198,199,473,474 CTs and other prospective studies with positive results24,185,187,198,199,346,356,358,397,469,473475 show cognitive improvement may be associated with five factors: Hcy state,37,57,200,358,397 disease duration,18,24,30,180,346,356 disease intensity,24,180,350,358,359 and treatment type,474,475 as shown in Table 4.17,24,37,58,146,180,290,306,346,350,351,353,356359,361,473495 The fifth factor, and perhaps the most striking, is whether or not the B12 deficiency-associated cognitive dysfunction is due to PA. If so, then these individuals may respond remarkably well to supplemental B12 treatment, regardless of the severity of the dementia.185,190,429,434 Parenthetically, all RCTs show supplemental folic acid is no better than placebo at improving cognitive function in individuals with dementia.348,355,361,496

Table 4
Factors associated with cognitive improvement in B12 supplementation of B12-deficient dementia

BPSD include symptoms and signs of disturbed behavior, mood, psychomotor activity, and thought content (eg, delusions and hallucinations) in patients with dementia. Exogenous SAM improves depression in patients with PD481 and other conditions.494,495 In individuals with AD, those with HHcy have increased EEG slow wave activity (ie, increased theta and delta waves) compared to those without HHcy.184,257 In nondemented individuals, lowering plasma Hcy levels with vitamin B12 treatment reverses such EEG findings.184,397 Accordingly, prospective studies of individuals with B12-deficient dementia show patients with delirium187 or psychoses359 improve with vitamin B12 supplementation. In demented individuals, prospective studies show vitamin B12 supplementation enhances vigilance when combined with bright light therapy,497 improves mood disturbances,475 but may worsen motor performance.353 Although not a consistent finding, 187,362 supplemental B12 in B12-deficient dementia may be beneficial for some patients with BPSD.359,475 Independent from serum folic acid and B12 levels, an association was found between HHcy and schizophrenia;498 in spite of the HHcy association being independent from group B vitamins in that study, another study198 showed combined pyridoxine, folic acid and vitamin B12 treatment in individuals with HHcy and schizophrenia improved positive symptoms, negative symptoms, and cognitive function. Depression, mania, psychoses, and delirium associated with PA or hypovitaminosis B12 may improve with supplemental B12 treatment.181184,190,191

Considering the potential risks for adverse events, added cost, and added administration, and the potential benefit for arresting or reversing dementia, if B12 deficiency is determined, it should be treated. Vitamin B12 treatment should be initiated in those cases where serum B12 levels are 200 pg/mL or less and those where serum B12 levels are 201–350 pg/mL with plasma Hcy greater than 14 μmol/L.320 Duration of therapy is based on etiology and other factors, with an optimum serum B12 not being more than 950 pg/mL. In cases of PA, treatment is continued for life. In cases of dietary deficiency, low vitamin B12 in dementia normalizes with dietary correction.14 Possible explanations on why dementia does not reverse or arrest with supplemental B12 include treatment being an ineffective form,146,290 initiation outside of a time course window,30,346,356 or dementia not solely being caused by PA or low serum B12.13,26,82,222,333,336,359,442

How is vitamin B12 deficiency treated?

Since bacteria produce natural forms of vitamin B12, which humans ingest by consuming animal products,19,58,201 strict vegetarians may require oral (PO) vitamin B12 supplementation.58 The recommended daily allowance (RDA) is 2.4 μg daily. 499 Supplemental forms of vitamin B12 include cyanocobalamin, used in the United States, hydroxocobalamin, used in Europe, and mecobalamin, used in Asia.7,361,475 Mecobalamin is a synthetic form of methylcobalamin, which is the type of cobalamin utilized intracellularly. Although it may be advantageous in cognitive improvement or protection,475 it has not been compared with cyanocobalamin or hydroxocobalamin in clinical trials, and it may not be readily available in many western countries. Routes for cyanocobalamin administration include PO, sublingual (SL), intranasal (IN), intramuscular (IM), and subcutaneous (SC). Although the intravenous (IV) route has been used in patients with renal failure,500,501 this route is not recommended for general use.448 Compared to PO cyanocobalamin, IM and IV cyanocobalamin potentially pose greater risks for anaphylaxis. Up to 98% of an IM dose is lost in the urine, and even more is lost with an IV dose.387,448 Alternatively, 1% of PO cyanocobalamin is absorbed, throughout the gastrointestinal tract, by passive diffusion, independent of intrinsic factor.18,52,201,368,387 Thus, 10 μg will be absorbed from a 1,000 μg dose, well above the RDA;18,201,502 this effect is seen even in those with PA, prior gastrectomies, or diseases of the terminal ileum.52,368,387,503 Most physicians favor IM over PO routes,20,23,31,203,504 while most patients favor PO over IM routes.503,505 Patient preference is supported by a CT91 and RCTs31,34,185,392 showing PO cyanocobalamin provides equal, if not greater, serum B12 therapeutic levels. Since PO cyanocobalamin is as effective as IM cyanocobalamin in treating B12 deficiency, and its complications, it may be the preferred route of administration, unless there is concern regarding PO cyanocobalamin adherence, such as dysphagia.19,200,376,505 Variance exists in recommendations for dose and administration frequency: common practices include cyanocobalamin 1,000 μg PO daily203,503,505 or cyanocobalamin 1,000 μg IM daily for one week, then weekly for one month, then monthly thereafter.23,185,200,203,346,356,387,393 Alternative regimens are recommended for those with manifestations of severe PA.448

Conclusions

Although this paper represents a synopsis of our current understanding between HHcy and hypovitaminosis B12 and MCI or dementia, some investigations and reviews reveal contrary results or recommendations.27,41,62,188,340,398,401,506508 Possible errors from this review and its conclusions are that Medline was the primary database, searches may not have captured all relevant studies and case reports, and it is presumed that there are unpublished vitamin B12 treatment responses. In order to attempt to minimize erroneous conclusions, results were crosschecked with Internet searches and basic and clinical science textbooks, other Medline searches were performed using various search terms, and communication took place with experts who care for demented patients. Owing to the latter, it is crucial that PA be ruled out in all demented patients. Also, the trials that have been reported to date may have been of insufficient size or duration to determine a beneficial effect.233

Biological data support the notion that ROS generation may lead to elevated Hcy, but this may be, at least in part, an adaptive mechanism to generate cysteine, which is used in the biosynthesis of glutathione,120,509 an antioxidant that protects cells from oxidative stress. Biochemical and epidemiological evidence is convincing that HHcy, with or without hypovitaminosis B12, is a risk factor for dementia. Evidence is less convincing that hypovitaminosis B12 is a risk factor for dementia in the absence of HHcy. B12 deficiency manifestations are variable and include abnormal psychiatric, neurological, gastrointestinal, and hematological findings. Radiological images of nondemented and demented individuals with HHcy frequently demonstrate leukoaraiosis, potentially related to BBB dysfunction. Thus, the finding of leukoaraiosis on CAT or MRI scan is an indication for checking plasma Hcy and serum B12 levels.

Although historically neuropathy related to PA was described according to a classical disease progression, patient samples now show that symptoms and signs secondary to the neuropathy can be variable.180,188 Nonetheless, dementia secondary to PA, occurring separately from other manifestations of SCD or peripheral neuropathy, and in the absence of macrocystosis and anemia, may still be considered sufficiently rare to be the basis of case reports.190,199,434

On the one hand, literature suggests assessing serum B12 and treatment of B12 deficiency is useful early in the course of cognitive dysfunction, for instance, MCI199,350,469 and mild to moderate dementia of less than two years duration.24,180,346,356,358,359 If low serum B12 is determined, it may be worthwhile to check for antiparietal cell and anti-intrinsic factor antibodies and obtain testing for H. pylori, for management and prognostic purposes. On the other hand, literature suggests assessing serum B12 and treatment of B12 deficiency may not be useful late in the course of cognitive dysfunction, for instance, moderately severe to severe dementia, if symptoms and signs of SCD and peripheral neuropathy, hematological manifestations of PA, and other factors listed on Figure 4 are absent. However, if one has not inquired about neurological symptoms, examined the mouth and tongue, performed a neurological examination, and obtained a CBC on a moderately-severe to severely demented patient, then it could be a ‘medical-legal pitfall’ not to check the serum B12 level.62 The suggestions for vitamin B12 workup and treatment in patients with suspected MCI or dementia are evidence-based. However, since the rate of dementia progression is poorly measured and variable and there are no specific criteria for vitamin B12 testing, the suggestions are not intended to replace care based upon clinical decisions. The serum B12 level remains the standard initial test.181 DAT is a compatible diagnosis when B12 deficiency is found, unless it is caused by PA. Cyanocobalamin PO is favorable over IM, in terms of both therapeutic efficacy and patient preference, unless there is concern regarding PO adherence.

Since the original observations by Biermer in 1872, to the more recent epidemic neuropathy in Cuba, it is evident that adequate folate and vitamin B12 function is required for physical and mental health. Issues surrounding folate, vitamin B12, Hcy, and MMA as they relate to health, especially microvascular disease and cognitive dysfunction, are high interest among researchers and clinicians. Puzzling questions remain. Since B12 deficiency can lead to depression, mania, and psychoses, should clinicians routinely check serum B12 levels in patients with such findings? Among those individuals who are B12 deficient, what are the prevalences of antiparietal, anti-intrinsic factor, and anti-H. pylori anti-bodies? Is HHcy a risk factor for other types of dementia, such as DBT, dementia due to PD, or dementia with Lewy bodies?193,510,511 Since HHcy has been prospectively shown to be an ‘independent’ risk factor for dementia, then why are Hcy-lowering therapies ineffective, or at best modestly effective, at preventing or treating dementia? Answers to questions such as these may one day unleash knowledge necessary for more effective treatment. The notions of age-related, and perhaps disease-related, degradation of the BBB and oxidative chemical reactions, generating ROS and elevating Hcy, coupled with immune activation, both without and within the brain, remain novel areas for further work.120

Table 3b
Studies showing vitamin B12 treatment is associated with improved cognitive function or prevention of dementia

Acknowledgments

The author wishes to thank the reviewers of this manuscript, for their thoughtful work and kind suggestions, which were highly valuable in the final writing of this publication. The author reports no conflicts of interest in this work.

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